The present invention relates to a vacuum brake booster and more particularly to a vacuum brake booster having a two-stage reaction arrangement.
From U.S. Pat. No. 2,826,041, a vacuum brake booster is known which comprises a constant atmospheric pressure chamber and a working chamber in which different pressures prevail, with a movable wall dividing the chambers and being mounted on a plunger, and a control valve which is actuated by a brake pedal. The control valve controls the differentials of pressure acting on the movable wall. Carried on the end of the plunger adjacent the control valve is a support plate whose outer edge is raised in the direction of the control valve. Between the plunger and the control valve, several reaction levers are positioned which, on the one side, bear against the movable wall radially outwardly and against a reaction-delaying spring radially inwardly, and, on the other side, bear against the edge of the support plate. In this arrangement, the point at which the reaction levers are in engagement with the support plate lies radially between the points of engagement on the spring and the movable wall. The spring is positioned between the control casing and the reaction levers and is adapted to bear upon the reaction levers directly, or through the intermediary of a disc. The biasing force of the spring provides for the what is termed "two-stage reaction" which means the retardation of the reaction force acting on the brake pedal.
In this known construction, the space available for the spring is small with regards to both the axial length and the diameter. Enlarging the space in a radial direction is not possible because this would reduce the lever arm through which the spring counteracts the reaction force. In addition, the point at which the spring is in engagement with the reaction levers must be placed in a smaller radius than the edge of the support plate. The use of a radially larger spring would, thus, require the whole lever mechanism to be enlarged radially, too, or the spring force would have to be shifted to a smaller radius by the insertion of an intermediary. An axial enlargement would result in an enlargement of the overall length of the brake booster.
The magnitude of the two-stage reaction is dependent upon the lever transmission ratio of the booster, the strength of the reaction-delaying spring and the point of impact of the reaction-delaying spring. Because of the small space available for accommodating the reaction-delaying spring, the magnitude of the two-stage reaction is limited.
Since only small springs may be used, there is the necessity of choosing a high spring stiffness which has, however, the disadvantage of the spring force increasing substantially when the spring is loaded. This means that the force of the spring is lower in the released state of the arrangement than during engagement of the valve piston with the reaction levers. The change of force is dependent upon the distance between the valve piston and the levers in the released state, which distance is largely affected by tolerances. These tolerances affect directly the magnitude of the two-stage reaction. Accordingly, it is not possible to obtain a defined two-stage reaction using a reaction-delaying spring having a high stiffness.
Moreover, the known arrangement results in lost motion in the reaction lever mechanism which has to be overcome first when the braking action is initiated.
Further, the use of strong springs raises assembly problems because the assembly of the individual parts which are in engagement with the reaction levers requires high expenditure of forces.